Process for wet-spinning fibers derived from acrylic polymers

ABSTRACT

A process is provided for producing &#39;&#39;&#39;&#39;as-spun&#39;&#39;&#39;&#39; acrylic fibers which have a void free, microporous well collapsed structure, great uniformity along the fiber axis, and a low degree of orientation. A process is also provided for producing acrylic fibers which can be converted to carbon and graphite fibers of high strength and modulus. The process involves the incorporation of certain additives in solutions of the polymer prior to spinning. Fibers obtained by this process may be stretched and hot drawn on further processing to develop maximum orientation with resultant excellent physical properties or they may be converted into high strength high modulus graphite fibers, by heating in an inert atmosphere at appropriate temperatures.

United States Patent 1 1 Morgan Feb. 18, 1975 [54] PROCESS FOR WET-SPINNING FIBERS 3,642,706 2/1972 Morgan. 264/184 DERIVED FROM ACRYLIC POLYMERS 3,681,275 8/1972 Takeya et al. 260/23 R [75] Inventor: Herbert S. Morgan, Apex, N.C. FOREIGN PATENTS OR APPLICATIONS 43-4549- 2/1968 Japan/"4.- 264/182 [73] Asslgnee' Mmsant Company i 45-19978 7/1970 Japan 264/182 [22] Filed: July 3,1972 [21] APPL Neil/58,356 Primary Examiner-Jay H. Woo

Related U.S. Application Data 57 ABSTRACT [63] Continuation of Ser. No. 115,861, Feb. 16, 1971, abandoned. A process is provided for producing as-spun acrylic fibers which have a void free, microporous well col- [52] U.S. Cl 264/182, 260/316, 264/29 lapsed structure, great uniformity along the fiber axis, [51] Int. Cl. D011 7/00 I and a low degree of orientation. A process is also pro- [58] Field of Search 264/ 182, 29, 184, 210 F; vided for producing acrylic fibers which can be con- 260/23 R, 31.6 verted to carbon and graphite fibers of high strength and modulus. The process involves the incorporation [56] References Cited of certain additives in solutions of the polymer prior to UNITED STATES PATENTS spinning. Fibers obtained by this process may be 3 329 557 7/1967 Magat et all 264,210}: stretched and hot drawn on further processing to de- 3:410:8l9 11/1968 Kourtz e:a1.IIIIII II 260/296 AB veloP maximum Orientation with resultant excellent 3,412,062 11/1968 Johnson et a1. 260 37 Physical Properties of y y be converted into high 3,464,922 9/1969 Bernholz et a1 252/86 strength g modulus g p fi y a g in an 3,556,729 1 1971 Holsten 23/2091 inert atmosphere at appropriate temperatures. 3,598,693 8/1971 Andersen et al..... 161/170 3,618,307 11 1971 Jonkoff 57 140 R 3 Claims, N0 Drawlngs PnocEss 1 FOR WET-SPINNING FIBERS DERIVED FROM ACRYLIC POLYMERS This is a continuation, of application Ser. No. 115,861 filed Feb. 16, 1971 and now abandoned. BACKGROUND OF THE INVENTION In the wet spinning process for forming acrylic fibers, a solution of the polymer is extruded through an orifice into a coagulation bath which usually contains a mixture of the solvent used to dissolve the polymer and a liquid miscible with the solvent such as water. The fiber is formed as the solvent diffuses out of the extruded polymer stream into the bath. This desolvation process results in a considerable concentration of the polymer during the conversion of the solution into filaments. The filaments are usually prevented from contracting during coagulation which means that there is effectively always some stretching of filaments during spinning. In addition to the stretching associated with desolvation there is usually a deliberate stretching imposed on the filaments by using a faster take-up rate than extrusion rate. Both stretching processes result in increasing the axial orientation of the polymer moleculesduring filament formation in the spin bath.

The terms spin orientation and jetstretch as used herein refers to orientation resulting fromthe shear effect produced by the flow ofa viscous solution through a spinnerette orifice plus the added orientation produced by the drag of the spin bath liquid on freshly coagulated filaments. Orientation in the spin bath hasbeen found to increase with spinning speed, polymer molecular weight (viscosity), solids content and adecrease in orifice size.

If the polymer solution is concentrated and gels rapidly, filaments are produced in which there is an outer gel layer surrounded by a liquid core which subsequently solidifies at a slower rate. This structure which occurs to a greater or lesser extent in all wet-spun fibers is known as the skin-core effect. The degree to which the skin-core effect takes place and the type of polymer structure is determined by the relative rates of diffusion of solvent and precipitation agent into the fiber and of polymer solvent from the fiber.

All wet spun acrylic fibers exhibitv some porosity in the micron range as a result of the coagulation process.

In most cases, however, these fibers include not only microscopic pores but also large macropores or voids. Y

It has been found that the degree of macroporosity is related to spinning conditions, increasing with nonsolvent concentration in the spin bath, bath temperature, orifice size and spinning rate, and decreasingwith polymer concentration in the spinning dope.

1n wet spinning commercial processes, it is highly desirable for reasons for economy and ease of spinnability to use high molecular weight, high concentration poly-. mer solutions and to spin these solutions at the highest speeds and using the largest number of jet holes possible. Unfortunately, these conditions also increase the tendency in a given acrylic polymer system toward spin orientation and void formation in the spin bath, and toward fiber non-uniformity. Further, these conditions also favor fibrillation of the fiber. The coagulation step in thespinning process is followed by another orientation step in which the fiber may be stretched in a conventional steam or hot water bath. Theorientation step mer solutions for which there is a needed improvement are (lithe instability of high solids polymer solutions toward aging, i.e., the pronounced tendency toward gellation on standing of these solutions, and (2) the difficulties involved in spinning void-free low denier fibers. The latter has become important at the present time with .respect to the use of acrylic fibers as precuF sors for theproduction of graphite fibers with high tensile strength and modulus.

In recent years, the need for materials having improved mechanical properties, such as tensile strength, stiffness, toughness and high-temperature strength has stimulated considerable interest inthe use of fiber reinforced resins. With the advent of the space age, a special need has arisen for low density, high corrosion and temperature resistant fiber reinforced composites havmeet the above requirements very effectively. Graphite is very corrosion resistant and is one of the few materials known whose tensile strength increases withtemperature. These fibers theoretically have the best properties of any known fiber for high strength reinforcement and in the future, graphite-reinforced composites may be used for many industrial high performance materials.

l-Iigh modulus carbon/graphite filaments are usually produced by the thermal decomposition of organicfilamentary precursors at high temperatures in a controlled atmosphere. Although several kinds of thermally stable" polymerssuch as polyvinyl alcohol, polybenzimidazoles, polyimides and aromatic polyamides have been used for producing carbon fibers, rayonand acrylonitrile fibers are by far the most widely used precursors. I i

In general, the conversion of any precursor yarn to a non-fused carbon or graphite yarn involves at least two and usually three distinct process stages each of which may be carried out in two or more steps as evidenced,

e.g., by U.S. Pat. No. 3,412,162. The first and by far the most time consuming of these is the preoxidation or filament stabilization stage, during which the polymers units may rearrange, the polymer is oxidized, hydrogen and other hetero-atoms are removed and some crosslinking may occur. This stage is essential so that the resultant yarn can .be further processed at higher temperatures without polymer burn out" taking place inthe filament core; Preoxidation is usually carried out by heating the substrate yarn in a gaseous oxidizing atmosphere at temperatures of less than 300 C. After substantially complete preoxidation, the yarn may be carbonized by a further heat treatment in the range of 800l400 C., in a non-oxidizing atmosphere, for a time sufficient to remove most of the hetero atoms,

- leaving a predominantly carbon fiber. If a graphite is generally known as the hot cascade stretch. The

yarn is desired, the fiber is then heated to 1,8003,000' C. in a non-oxidizing atmosphere in order to effect graphitization. At these temperatures,

v bond rearrangements occur and graphite crystallites form and grow.

In order to obtain a high strength, high modulus lites in the yarn must be oriented largely parallel to the longitudinal axis of the fiber. In the case of acrylic yarns, in which the polymeric carbon backbone can theoretically remain largelyintact during the overall conversion to graphite, some degree of the orientation shown in the final graphite yarn may be developed at any stage ofthe process, including the spinning and drawing of the precursor yarn. The application of stress, such as by stretching at one or more stages, while the yarn is being heated to graphitization temperatures is essential to the development of the orientation and crystallinity necessary for high strength and modulus. When a batch process is used, stress may be developed in the fiber by internal shrinkage if the fiber if the fiber is maintained at constant length during processing.

When untreated acrylic yarns are subjected to differential thermal analysis (DTA), a large exotherm begins around 200-230 C. and peaks at 300320 C. When such fibers are heated in air to temperatures above about 230 C., thermally initiated, highly exothermic reactions occur which release sufficient heat to result in inter-filamentary fusion and embrittlement. Extensive chain scission and polymer decomposition also occur resulting in the formation of a hard and brittle char which is not a satisfactory form of graphite precursor fiberl The necessity for heating PAN fibers for long periods of time at relatively low temperatures, as required by prior art processes for preparing satisfactory graphite precursors by air preoxidation thus becomes apparent. Usually the preoxidation is carried out in two steps. In the first step, the temperature is held below 230 C. for the length of time required for the exothermic reactions to become essentially complete at rates such that the heat can be efficiently transferred to the surrounding medium. After completion of the first step, the temperatures may be increased to 300 C. or greater, in order to speed up the oxidation, remove most of the remaining hydrogen and flameproof the fiber for crabonization and graphitization. If the temperature is raised above a certain value too rapidly, uncontrollable exothermic reactions will take place. At lower temperatures, the desired stabilization reaction, with air, which is diffusion controlled will take place rapidly at the filament surface and at progressively slower rates with increasing distance from the surface. If the fiber is heated to a high temperature, prior to complete stabilization of the substrate polymer in the core, decomposition of the undertreated fiber interior will occur and result in the formation of holes or macrovoids in the core as well as hard char carbon, both of which are detrimental to good fiber properties.

A distinct drawback, however, exists in the tension preoxidation process as heretofore practiced. As indicated in US. Pat. No. 3,412,062, the time required for preoxidation, i.e., complete permeation of oxygen throughout the yarn, is quite lengthy/For example, the patentees state that 24 hours is required for the preoxidation of a 2.5 denier at 220 C. and even longer peri-. ods of time for larger denier fibers. It would, of course,

be highly desirable to shortenthe period of time required for preoxidation and at the same time produce preoxidized yarns which, when carbonized and/or graphitized, retain their structural integrity and yield high modulus high strength graphite fibers.

A consideration of the various possible reactions involved suggests that better control of the preoxidation process may be obtained and the rate of conversion in creased if the fibers could be stabilized against the highly destructiveexothermic reactions which occur during the preoxidation stage. Further, better control of the preoxidation process may be obtained if spinning conditions are chosen which result in as spun fibers having uniform dimensions and properties along the fiber length, a low degree of fibrillar orientation and a uniform void-free fine structure comprised of micropores which are interconnected and which communicate with the fiber outer surface. The fibers described above could be drawn and oriented during subsequent processing to yield high strength, high modulus graphite fibers. v

Although thedemand for uniform, high modulus, high strength graphite fibers has grown rapidly, the known methods for producing the same has remained time consuming and costly. Most of the prior art methods for producing graphite fibers from acrylic precursors require long processing times to carry out the preoxidation step. In addition, the precursor acrylic fibersusedin such processes are usually obtained by conventional wet-spinning methods and are of inferior quality for graphitization use due to preorientation, non-uniformity of thefiber dimensions andstructure, and the presence of imperfections, such as voids.

Accordingly, it is an objective of the present invention to provide a simple, efficient and economical method for the preparation of acrylic fibers having a void free, well collapsed structure, greater uniformity and a lower degree of fiber orientation.

Another object of this invention is to provide a method for obtaining low denier, uniform acrylic fibers at high rates of productivity.

Another object of this invention is to provide a method for obtaining non-fibrillating acrylic fibers.

Another object of the invention is to provide a simple, efficient method for the manufacture of high strength, high modulus graphite fibers derived from acrylonitrile polymers or copolymers.

" Yet another objective of the invention is the provision of a method for the production of a preoxidized acrylic yarn of the preferred structure for subsequent conversion to high strength, high modulus graphite yarn.

Other objectives will become apparent hereinafter.

SUMMARY OF THE INVENTION In accordance with this invention there is provided a process for spinning dense, substantially void-free, uniform acrylic fibers which comprises providing a-solution of acrylic polymer in a solvent therefor which solution contains between about 0.1 and 5.0 percent of an as u ' polyacrylonitrile as well as copolymers and terpolymers of such polymers and their conversion into fibers are well known by those skilled in the art.

It is well known by those skilled in the art that spinning variables which affect fiber fine structure, orientation and uniformity are the size and number of holes in the spinnerette, bath temperatureand concentration, rate of spinning, jet stretch, cascade stretch, washing and drying efficiency and draw ratios and temperatures. Polymer variables which are known to affect spinnability and fiber properties are composition, concentration, viscosity (or molecular weight) and dope temperature. It is also well known that all of the abovementioned variables are mutually interacting and a choice of optimum spinning conditions for a given polymer composition must be made with respect to a combination or set of conditions, rather than variation of a single spinning condition.

For purposes ofthe initial investigation leading to thepresent invention and for detailed studies of permissable variations of the reaction conditions involved therein, fibers derived from a copolymer composed of 93 mole percent acrylonitrile acetate were used.

Numerous attempts were made to obtain microporous, unoriented void free fibers from the above polymer composition by wet spinning. Spinning conditions which were varied in those experiments were orifice size, concentration and temperature of the spin bath, spinning rate, immersion length and jet stretch. In all of the many various combinations of spinning conditions employed the fibers obtained had inferior structural quality and would not be stretched or drawn to the extent desired to develop optimum properties in either the cascade stretch or hot-draw stages of the spinning process. X-ray diffraction patterns, birefringence measurements and fiber cross sections were obtained on fiber samples spun under different conditions and taken at various points in the spinning process. A consideration of the results led to the conclusion that orientation of the fiber in the spin bath, as well as void formation, had an adverse effect on spinnability, cascade (wet) stretch and hot (dry) stretch and thus on the fine structure and properties of the resulting fibers. Further, the amount of spin orientation and void formation are related to the coagulation rate, diffusion rate and interchange of solvent and water within the fiber gel structure during coagulation. The major factors controlling the coagulation and diffusion rates at constant denier would appear to be the dope temperature; spin bath concentration, temperature and immersion length, and the calculated jet stretch-for a particular orifice size.

The attainment of a high total stretch with minimum fiber orientation during the coagulation and cascade stretch stages of the wet spinningprocess is essential in order to obtain well collapsed void-free fibers having excellent physical properties. Excessive orientation in the freshly coagulated filaments during the spinning process is believed to be responsible for the abovenoted inferior structural quality of fibers obtained under standard spinning conditions, and logical variations thereof.

and 7 mole percent vinyl I It has now been found that excessive orientation in the as spun fibers can be minimized by the addition of certain additives to these acrylic polymer solutions prior to spinning. The presence of these additives in the dope, which is acritical and necessary part of this invention, is believed to prevent the polymer solution from coagulating too rapidly in the spin bath. Although the exact mechanism is unknown, it is believed that the additives of this invention reduce the rate of coagulationby controlling the rate of diffusion of water and solvent into and from the partially coagulated fiber. which is maintained in the amorphous state. As a result of the controlled coagulation and diffusion rates, a void-free, uniform, fine structure is attained in the freshly coagulated fiber. Further, fibers obtained by the coagulation process described above have been found to be less fibrillating whereas those obtained by standard spinning processes known to the art will fibrillate unless special treatments, such as heat treatment by annealing is applied.

Solutions of these polymers which contain no additives are believed to coagulate rapidly at the surface in the near vicinity of the spinnerette resulting in the formation of a skin-core. The rapidly coagulated skin is believed to reduce the diffusivity of water into the fiber and inhibit the removal of solvent therefrom. The differing rates of coagulation between the outside layer and inner core of the fiber are believed to be responsible for thedevelopment of variable spin orientation in the fiber in the spin bath. As a result, fibers emerging from the bath have a higher degree of orientation and structural variability. As spin orientation increases, fiber structure becomes poorer and the maximum attainable orientation stretch is reduced. The addition of the additives of this invention to such dopes results in a reduction of spin orientation and consequently improved fiber fine structure and uniformity. As gel fiber structural uniformity is increased, the maximum attainable stretch is increased with a resulting improvement in fiber properties. Since orientation has been found to increase with spinning speed, polymer molecular weight, solids content and a decrease in orifice size, the

advantage of using the additives of this invention is obvious.

In addition to the pronounced reduction in nonuniform orientation and void formation, and the improved fiber uniformity and fine structure, the additives of this invention have been found to improve the stability of acrylic polymer solutions or slurrys toward gellation on aging and heating. Since dope stability is known to decrease with increased molecular weight and polymer concentration, it follows that one of the major advantages offered by the use ofthe additives of this invention is a longer pot life and/or the use of higher polymer concentrations.

The additives that may be used in the practice of this invention are the polyalkoxylated fatty acid esters of sorbitol, such as for example, polyoxyethylene sorbitan monopalmitate, polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitan mono-oleate, polyoxyethylene sorbitan monolaurate. polyoxypropylene sorbitan monopalmitate, polyoxybutylene sorbitan nonopalmitate, and the like.

Y The alkoxy portion of the additive used in the practice of this invention is composed of alkoxy radicals containing from one to five carbon atoms. That is, oxymethylene, oxyethylene, oxypro'pylene, oxybutylene and oxypentalene. The polyalkoxy should contain from between about 4 to about 40 moles, i.e., recurring groups of the alkoxy radicals in the polymer chain. Less than about 4 moles or more than about 40 moles has been determined to be ineffective in attaining the objects of this invention inasmuch as the high jet and/or cascade stretch needed are not effective and consequently the fibers produced do not have the desired uniform, fine structure.

It is preferred that the additive employed in the process of this invention be a liquid and water soluble, both for ease of operation and for attainment of the stated objects. The fatty acid portion of the additive preferably contains between five and 20 carbon atoms. These acids are well known in the chemical arts as is the method of formation of the ester with sorbitol and the alkoxylation procedure.

The concentration of the additive used is in the range of from about 0.1% to about 5% and preferably from about 0.5% to about 1% by weight based on the weight of polymer in the solution. The optimum amount of additive is dependent on the spinning parameters employed such as rate of spinning, dope temperature, spin bath concentration and temperature, fiber denier and number of filaments and polymer variables such as composition, molecular weight and concentration. The optimum amount of additive may be easily determined depending on the desired result. It should be kept in mind that the optimum conditions used in the practice of this invention will be governed by the desired properties, denier, and end use of the final product, which may be varied as desired within broad limits by controlling the spinning and drawing conditions used.

The method used for additing the additives of this invention to the polymer solution is not critical. For convenience and ease of mixing, the additives may be dissolved in a small amount of the solvent used for dissolving the polymer and added to the polymer slurry or solution while stirring.

Coagulation baths suitable for converting viscous polymer solutions to fibers are well known and are usually comprised of water or a mixture of water and a lower alkylamide solvent. The preferable spin bath composition may vary over a wide range which will be mainly dependent upon the composition of the polymer, the number and size of the filaments and the use for which the fiber is being produced. For ease of solvent recovery, it is suggested that the solvent used in making up the spin bath sou ld be the same as that used for dissolving the polymer.

In. order to maintain an essentially constant environ ment for coagulation in the bath, the solution should be circulated and maintained at the optimum concentration by the continuous addition of water while maintaininga constant volume of liquid bath coagulation media. The temperature of the coagulation bath may be from about to about 50 C., and preferably from about to C. during coagulation.

The rate of production or spinning speed may be varied from about 50 feet per minute to about feet per minute. The optimum rate for a given system will be determined by the denier and total number of filaments; polymer variables, such as molecular weight; composition, viscosity and the'fiber properties desired.

When coagulated in the manner described above, the

which permits their conversion by further processing into useful, non-fibrillated acrylic fibers having unique and novel properties. Additionally, they may be converted by standard means to graphite fibers having superior tensile strengths and moduli.

The coagulation step in the process is followed by an orientation step in which the fiber is stretched from about one to about six times its length in a conventional hot water or boiling water stretch bath. The orientation step is generally known as the hot cascade" stretch.

Fibers having a denier of l d.p.f. or less may be obtained by the process of this invention. whereas. when conventional spinning techniques are employed without the additives of this invention, uniform fibers of less than 3 d.p.f. are very difficult, if not impossible, to obtain, particularly in a wet spinning process. One of the major advantages of the process of this invention is that is makes possible the spinning ofa large number of filaments in a towwith minimum sacrifice of fiber uniformity.

As mentioned previously, the fibers obtained from the hot cascade stretch may be processed further by standard means to give acrylic fibers having improved properties or they may be converted to graphite fibers having improvedproperties by means known to the art.

Although no means short of actual conversion to graphite fibers can be relied upon as an indication of the suitability of a given precursor fiber for graphitization, there are two tests which can be generally depended upon to give a qualitative measure of the suitability of the microstructure of the fiber for graphitization.

In the first of these, several wraps of fiber are wound around a thermocouple lead which is inserted in an oven maintained at a constant temperature, such as 230 C., and a plot of the temperature of the'fiber bundle versus time recorded. The experiment can be repeated at higher or lower temperatures. In this way, the temperature at which an exotherm occurs and the height of the exotherm (Al) in degrees can be ascertained. It has been found that fibers which exhibit a minimum exotherm rise can be converted more rapidly into carbon or graphite fibers having improved properties.

The second method involves immersing a known weight of freshly spun fiber into a basic dye bath of known concentration and measuring the amount of dye uptake by the fiber. The more open the micropore structure and lower the orientation, the greater will be the dye uptake for a given fiber and the more readily will the fiber be preoxidized and converted into carbon form. The basic dye uptake can be used in connection with the exotherm or heating experiment described previously.

Fibers prepared by the process of this invention may be preoxidized or heat stabilized by such well known conventional processes as heating in air described in U.S. Pat. No. 3,412,062 or by the bromination process described in U.S. Pat. No. 3,556,729 or the process described in copending application Ser. No. 72,223 filed well collapsed, uniform, void-free fibers emerging from the spin bath have a low birefringence, or orientation,

Sept. 14, 'l 970, involving the treatment of the fiber with a hot solution of a basic catalyst in a polyol at a temperature and residence time sufficient to stabilize the fiber, followed by completion of the preoxidation by treatment with air at elevated temperatures, e.g., above 270 C.

When the preoxidation is carried out in air,- the conditions of time, temperature, rate of heating, and exposure to the'gaseous oxidant can be varied, and the'combination of conditions necessary to give optimum re- 10 PREFERRED EMBODIMENTS EXAMPLE I This example is presented to describe a typical'conventional wet spinning process for the production of DESCRIPTION'OF ofthe original fiber length is the minimum amount applied.

suits for Specific e sample'derived from a given 5 acrylic fibers with no additive. While this andother ex- Polymer System readly e m Precursor yams amples refer to a specific acrylic copolymer it is to be ee by the Process of thlemvemwn may be-preox understood that the procedure is essentially the same ldlzeemore rapldly and h'gher temperatures than and the invention operative with all types of acrylic yarns prepared-by conventlonal processes due to the polymers mentionfid above 4 lack of the characteristic destructive exotherm exhlb- A slurry is prepared containing 2 500 grams y these fibers; acrylic polymer, composed of 93% acrylonitrile and 7%- While the formation of a suitable stab1l1zed fiber can Vinyl acetate in 7500 grams of N I be Oetamed by Pf eombmanons P e dimethylacetamide (DMAC) solvent and is chilled to ment1oned cond1t1ons over a board range, it is obvious w rapid stirring After thorough mixing is that relatively high temperatures m 'P i h complished, the stirring is slowed and the temperature shorter periods of time will be preferred 1n most inallowed to rise to o o C after which the mixing stances- Howeveri e e il g vessel is heated to raise the temperature to 80 C temperature treatment celldmons must e 0.05% or whereby. the slurry becomes a clear viscous solution. a gven polymer eomposmon and l physlcal form The solution is spun througha horizontally placed such that the rate of transformation Wlll not occur so 1 440 hole spinnerette into a coagulating b'ath rapidly as to result in the occurence of physical disrupposed of 53% DMAC and water." bath has a of he fiber P undeslrable Slde reactions temperature of 35 C. and the spinnerette pump rate is i addmon to tune temperature and pglymer e adjusted to 202cc. per minute. This pump rate results smon other faeters affeetmg the e e converse, of in a calculated jet stretch of 1.29. with the first godet a yarn to the desired degree of stabilization when using roll having a speed of 13 feet per minute The gelled the polyol-basic catalyst processare the composition of fiber is wash-ed with water at 0 on the first godet the heating bath, concentration of the base catalyst, the then stretched 53x which, was the maximum amount e tension and the phyeleal paremeters the fiber of stretch attainable, through a boiling water cascade belng treated Such as denler, y P W and h bath. The yarn is then washed at constant length before ratlovof Surface area to volurne' being dried at 1 10 C. at constant length and taken up e usmg the Polyolbasle eatalyst process e on.a bobbin. The yarn bundle had a total denier of bilizatron or preoxldation, tension must be apphed to 2,160 or abeut 150 denier per fiber (dpi), with a I the fiber to preyent substanfial shrlnkage during bOth' cit'y of 3.8 grams/denier and an elongation of 11%. 'steps of the stablllzatlon. Tl'llS produces stabilized fibers The acrylic fiber exhibited a basic dye acceptance of which may be graphitized into fibers which have high strength and modulus. I

Acrylic polymer yarns prepared by the process of this EXAMPLES H Vl invention, and stabilized or preoxidized according to The procedure of Example I was repeated with the one of aforementioned procedures are in a preferred variances noted in Table I below and with the addition form for direct conversion, without further carboniza- 40 to the polymer slurry of 0.5%, based on the polymer tion to high strength, high modulus graphite yarns by weight, of polyoxyethylene v(2O) sorbitanmonopalmimeans of any conventional procedure and apparatus tate. The results are set out in Table 1.

TABLE I Polymer Cascade Solids Spin Bath Solvent Calculated Stretch Hot Stretch Denier/ Tenacity Elongation Basic Dye Ex. Concentration .let Stretch at Boil at C. dpf g.p.d. Acceptance 11 '20 57 1.0 2.30 4715/3117 1.7 17.5 6.4 III 20 57 2.0 4.77 1.85 642/045 4.1 8.0 3.4 IV 24 70 1.0 7.69 1757/1.22 4.1 14.0 4.8 V 24 70 1.0 7.69 1.46 12lO/O.84 5.7 8.0 3.9 VI 24 70 2.5 2.30 2413/l.67 2.2 15.0 6.0

used for this purpose. Graphitization of these preoxi- It can be readily seen from the results above that the dized yarns may be attainedv by' heating them to 1 use of the additive in accordance with this invention l,800-3,000 C. in an inert atmosphere, e.g., in the shows a marked improvement in the fiber structure. presence of argon and preferably while they are under This is especially apparent from the dye acceptance, all tension. 60 of which are substantially greater than the 2.0% dye ac- The application of longitudinal tension during preoxceptance of the fiber of Example 1 which had no addiidation is known to greatly increase the strength and v tivc. Further, Table 1 shows that the use of an additive modulus of carbon and/or graphite fibers produced in accordance with this invention allows greater stretch from such preoxidized fibers. Generally, tension suffiratios and permits the production of acrylic fibers at cient to limit the shrinkage ofthe fiber to not more than 6 lower deniers than heretofore though possible by a conventional wet spinning process.

EXAMPLES vn AND VIII Two additional acrylic fiber samples are prepared in accordance with the procedure outlined in Example I with the exception that each had 0.5% by weight, based on polymer weight, of polyoxyethylene sorbitan monopalmitate added to the polymer dope. After being taken up, the yarn was converted into graphite by first passing the yarn bundle through a bath of glycerine,

containing 0.03 me/g of sodium glyceroxide as stabilization catalyst. The bath was held at 200 C. and the residence time of the yarn in the bath was 4 minutes. Afterward, the yarn was heated in air at 310 C. for 45 minutes and thereafter passed through a graphitization furnace heated to 2,700 C. with a residence time in the furnace of 1 minute. The yarn produced in Example I is similarly graphitized and the results are given in Table ll.

'nerette into a coagulating bath maintained at n temperature of from about 20 C to about 50 C. stretching said fibers from about one to six times their length in a boiling water bath and thereafter collecting the fibers on collecting means.

2. A process according to claim 1 wherein there is added between about 0.5% and 1.0% of the pol'yalkoxylatcd fatty acid ester of sorbitol.

TABLE ll Example Total Denier/ Graphitization Density Tensile Strength Sonic No. of Filaments Furnace Tension P S.l X 10 Modulus P.S.l. X 10 Vlll8l6/l440 500 grams 1.83 413 55 VIII l792/l440 400 grams L88 354 54 maximum 1.71 246 47 Table II clearly indicates the substantial improvement in the'properties of graphite fibers fabricated 3. A process according to claim 1 wherein there is added to the solution between about 0.l% and 5.0%,

from the acrylic yarns of this invention over those conbased on polymer weight, of polyoxyethylene sorbitan ventionally employed in the production of graphite fibers.

monopalmitate. 

1. A PROCESS FOR PRODUCING ACRYLIC FIBERS HAVING AN IMPROVED FINE STRUCTURE AND OUTSTANDING SUITABILITY FOR USE A GRAPHITE PRECURSORS WHICH COMPRISES: SOLUTION OF AN ACRYLIC POLYMER IN A SOLVENT THEREFOR, ADDING TO THE SOLUTION FROM ABOUT 0.1 PERCENT TO 5.0 PERCENT BASED ON POLYMER WEIGHT, OF A POLYALKOXYLATED FATTY ACID ESTER OF SORBITOL, FORMING FIBERS FROM SAID SOLUTION BY EXTURDING THE SOLUTION THROUGH A SPINNERETTE INTO A COAGULATING BATH MAINTAINED AT A TEMPERATURE OF FROM ABOUT 20*C TO ABOUT 50*C, STRETCHING SAID FIBERS FROM ABOUT ONE TO SIX TIMES THEIR LENGHT IN A BOILING WATER BATH AND THEREAFTER COLLECTING THE FIBERS ON COLLECTING MEANS.
 2. A process according to claim 1 wherein there is added between about 0.5% and 1.0% of the polyalkoxylated fatty acid ester of sorbitol.
 3. A process according to claim 1 wherein there is added to the solution between about 0.1% and 5.0%, based on polymer weight, of polyoxyethylene sorbitan monopalmitate. 